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Horseweed, giant ragweed and common ragweed have been confirmed glyphosate-resistant in Indiana and now glyphosate-resistant waterhemp can be added to the list. In the summer of 2009, seeds of two suspected glyphosate-resistant waterhemp populations (WR1 and WR2) were collected from southwest Indiana soybean fields and screened for resistance in greenhouses at Purdue and Southern Illinois University-Carbondale. Plants ranging from 3 to 5 inches in height were sprayed with glyphosate at 0.75 and 2.25 lbs of ae/a. At 21 days after application (DAT) individual plants were rated as dead or alive and percent survival of the two suspect populations were compared to known resistant and susceptible biotypes.

Figure 1. Control of glyphosate susceptible and resistant waterhemp at 21 days after treatment.

Glyphosate has generally been an effective herbicide for control of waterhemp. However, control can be variable at times. In the known sensitive population, 0.75 lb ae/A of glyphosate killed 80% of the plants and 94% were killed with 2.25 lbs of glyphosate. The resistant biotype displayed some stunting and chlorosis, but only 10% were killed with either rate of glyphosate and 90% survived. The WR1 population had survival rates of 92% and 57% respectively, with the 0.75 and 2.25 lb rates. Percent survival in the WR2 population was 64% and 33%, respectively. Plants from WR1 and WR2 survived applications that controlled the susceptible biotype indicating glyphosate resistance is present in both populations. Further testing will be performed on future generations to determine the inheritance and level of glyphosate-resistance in these particular populations.

Glyphosate can be used to effectively control glyphosate-susceptible waterhemp, but not glyphosate-resistant populations. Alternative methods will be needed to control glyphosate resistant populations. Researchers in Missouri and Illinois have isolated waterhemp populations that are resistant to glyphosate and PPO (Cobra/Flexstar), PSII (Sencor), and ALS (Classic/FirstRate) inhibiting herbicides. Weed Scientists sarcastically call these populations double, triple, or quadruple stacks!

The bottom line is that we now have glyphosate resistant waterhemp in Indiana. The populations discussed here are in Vigo county in the southwest part of the state. We have significant waterhemp infestations in southern Indiana, northwest Indiana and east central Indiana as well. It would not surprise us if eventually we have glyphosate resistant waterhemp populations in those areas as well if we don’t change management tactics. For more information on control of glyphosate-resistant waterhemp please check out the extension publication ‘Biology and Management of Waterhemp’ at http://www.glyphosateweedscrops.org/. We would also like to thank Bryan and Julie Young at SIU for providing seed to use in our greenhouse screening experiments and for running additional screening experiments to confirm that our populations are glyphosate resistant.

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